Our application addresses the major medical problem of hepatocellular carcinoma, the third leading cause of cancer death worldwide. This cancer is understudied compared to other major lethal cancers and is in desperate need of new therapies. The only current treatments that prolong survival are transplantation and surgical resection/ablation, and these are effective only in a minority of patients. Moreover, hepatocellular carcinoma is inherently chemoresistant and no drugs have been shown to improve survival. To develop more effective therapeutics we need a better understanding of the underlying molecular alterations that drive HCC development and thoroughly test their functional significance. Towards this end, we have assembled a research team with a long history of collaboration in the area of cancer target discovery and validation to identify and characterize new oncogenes and tumor suppressor genes involved in HCC. Our approach integrates a series of powerful technologies, all developed at Cold Spring Harbor Laboratory, that both identify and prioritize genomic alterations in HCC, and provide tools to thoroughly study their biological relevance to cancer. Potential oncogenes and tumor suppressor genes are identified by a sensitive and high-resolution array method and then prioritized for their likely relevance by cross-species comparison and expression studies. High priority candidates are tested and validated using a highly tractable mouse model of HCC that allows for rapid analysis of genes and gene combinations. Validation approaches exploit new RNA interference technology to both conditionally and stably silence gene expression in vitro and in vivo. We provide an extensive series of preliminary results that have validated this approach, including demonstration that human and murine HCCs have common genetic lesions and the development of a high throughput model of murine HCC. Finally, we have used this approach to identify and validate clAP1 as a new oncogene relevant to HCC. Our results will produce new insights into the molecular basis of hepatocellular carcinogenesis and should identify key therapeutic targets for this highly lethal cancer.